linux-kernel - Re: [PATCH] mm: softdirty: write protect PTEs created for read faults after VM

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Message-ID: <20140821205115.GH14072@moon>
Date:	Fri, 22 Aug 2014 00:51:15 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Peter Feiner <pfeiner@...gle.com>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Pavel Emelyanov <xemul@...allels.com>,
	Jamie Liu <jamieliu@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Magnus Damm <damm@...nsource.se>
Subject: Re: [PATCH] mm: softdirty: write protect PTEs created for read
 faults after VM_SOFTDIRTY cleared

On Thu, Aug 21, 2014 at 03:37:37PM -0400, Peter Feiner wrote:
>
> Thanks Kirill, I prefer your approach. I'll send a v2.
> 
> I believe you're right about c9d0bf241451. It seems like passing the old & new
> pgprot through pgprot_modify would handle the problem. Furthermore, as you
> suggest, mprotect_fixup should use pgprot_modify when it turns write
> notification on.  I think a patch like this is in order:
> 
> Not-signed-off-by: Peter Feiner <pfeiner@...gle.com>
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index c1f2ea4..86f89a1 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1611,18 +1611,15 @@ munmap_back:
>  	}
>  
>  	if (vma_wants_writenotify(vma)) {
> -		pgprot_t pprot = vma->vm_page_prot;
> -
>  		/* Can vma->vm_page_prot have changed??
>  		 *
>  		 * Answer: Yes, drivers may have changed it in their
>  		 *         f_op->mmap method.
>  		 *
> -		 * Ensures that vmas marked as uncached stay that way.
> +		 * Ensures that vmas marked with special bits stay that way.
>  		 */
> -		vma->vm_page_prot = vm_get_page_prot(vm_flags & ~VM_SHARED);
> -		if (pgprot_val(pprot) == pgprot_val(pgprot_noncached(pprot)))
> -			vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> +		vma->vm_page_prot = pgprot_modify(vma->vm_page_prot,
> +		                        vm_get_page_prot(vm_flags & ~VM_SHARED);
>  	}
>  
>  	vma_link(mm, vma, prev, rb_link, rb_parent);
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index c43d557..6826313 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -324,7 +324,8 @@ success:
>  					  vm_get_page_prot(newflags));
>  
>  	if (vma_wants_writenotify(vma)) {
> -		vma->vm_page_prot = vm_get_page_prot(newflags & ~VM_SHARED);
> +		vma->vm_page_prot = pgprot_modify(vma->vm_page_prot,
> +		                       vm_get_page_prot(newflags & ~VM_SHARED));
>  		dirty_accountable = 1;
>  	}

Thanks a lot Peter and Kirill for catching it and providing the prelim. fixup. (Initial
patch doesn't look that right for me because vm-softdirty should involve into
account for newly created/expaned vmas only but not into some deep code such
as fault handlings). Peter does the patch above helps? (out of testing machine
at the moment so cant test myself).
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